In recent years, information electronic devices such as a personal computer, a cell phone and a PDA and audio-visual electronic devices such as a video camcorder and a minidisc player have been developed to be compact, light and cordless. With this development, demand is increasing for secondary batteries with high energy density as a power source for these devices. Under such circumstances, the commercialization of non-aqueous electrolyte secondary batteries with higher energy density than conventional secondary batteries such as lead acid battery, nickel cadmium battery and nickel metal hydride battery is proceeding.
Non-aqueous electrolyte secondary batteries as typified by a lithium ion secondary battery and a lithium ion polymer secondary battery use, as a positive electrode active material, a transition metal oxide with an average discharge potential of 3.5 to 4.0 V relative to that of metal lithium, such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2) and spinel structure type lithium manganese oxide (LiMn2O4), or a solid solution material incorporating a plurality of transition metals (LiCoxNiyMnzO2, Li (CoaNibMnc)2O4). They are used singly or in combination thereof. These active materials are mixed with a conductive material and a binder, which is then applied onto a current collector made of, for example, aluminum, titanium or stainless steel, followed by rolling to give a positive electrode.
As for a negative electrode, a carbonaceous material capable of absorbing and desorbing lithium ions is commonly used. As the carbonaceous material, there are used artificial graphite, natural graphite, graphitized mesophase carbon made from coal pitch or petroleum pitch, non-graphitizing carbon (hard carbon), etc. They are used singly or in combination thereof. These carbonaceous materials are mixed with a binder and the like, which is then applied onto a current collector made of, for example, copper, iron or nickel, followed by rolling to give a negative electrode.
A negative electrode using a graphite material typically has an average potential allowing release of lithium ions which is 0.2 V lower than that of a negative electrode using non-graphitizing carbon (hard carbon). Accordingly, graphite materials are mostly used in the field which requires a high voltage and flat plateau in voltage characteristic.
A non-aqueous electrolyte is desired to be proof against an oxidizing atmosphere of the aforesaid positive electrode which discharges at a potential as high as 3.5 to 4.0 V relative to a potential of metal lithium and against a reducing atmosphere of the negative electrode which charges and discharges at a potential close to that of metal lithium. Currently, a non-aqueous electrolyte is prepared by dissolving lithium hexafluorophosphate (LiPF6) in a non-aqueous solvent obtained by mixing ethylene carbonate with a high dielectric constant (hereinafter referred to as “EC”) with a linear carbonate with low viscosity such as diethyl carbonate (hereinafter referred to as “DEC”), dimethyl carbonate (hereinafter referred to as “DMC”), or ethylmethyl carbonate (hereinafter referred to as “EMC”).
Such non-aqueous electrolyte, however, contains a linear carbonate whose viscosity is low and boiling point is around 100° C.; therefore, it has a high vapor pressure at high temperatures, which may cause the battery itself to expand. Moreover, since LiPF6, which is thermally unstable and prone to hydrolysis, is used as a solute, a gas is likely to occur inside the battery, promoting the expansion of the battery.
In view of the foregoing, studies have been carried out on a lithium salt as an alternative to LiPF6. For example, the use of LiBF4, lithium bis(perfluoromethylsulfonyl)imide (LiN(SO2CF3)2, hereinafter referred to as “LiTFSI”) or lithium bis(perfluoroethylsulfonyl)imide (LiN(SO2C2F5)2, hereinafter referred to as “LiBETI”), all of which are more thermally stable than LiPF6, reduces the ionic conductivity of a non-aqueous electrolyte and thus lowers the discharge characteristic of a battery. Further, LiTFSI has the problem that it corrodes aluminum, which is mostly used as a current collector for a positive electrode, at a high potential of 3.7 V or higher relative to a potential of metal lithium. The use of LiBETI improves the corrosiveness; however, because it has a large molecular weight, it is likely to increase the viscosity of a non-aqueous electrolyte.
Recently, lithium bis(fluorosulfonyl)imide is being developed as an imide salt (see, for example, Japanese Laid-Open Patent Publication No. Hei 8-511274).
In order to prevent the vapor pressure from increasing at high temperatures, the use of a solvent with a high boiling point such as propylene carbonate (hereinafter referred to as “PC”) or γ-butyrolactone (hereinafter referred to as “GBL”) is considered instead of using the linear carbonate with a low viscosity and a low boiling point. GBL, however, reacts with LiPF6 at high temperatures, thereby increasing the polarization resistance of a battery to decrease the charge and discharge characteristics.